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Chapter 2

Protocol Attributes

Before we go any further with peer initialization, we need to discuss sev-

eral terms that are specific to OSPF. These terms are key to understanding
OSPF and how it functions.

The easiest way to understand OSPF is to build from what you already

know about EIGRP. You learned that EIGRP uses autonomous system num-
bers to specify routing processes and the routing process that individual
routers belong to. OSPF uses areas in place of an autonomous system. An
OSPF area consists of a group of routers or interfaces on a router that is
assigned to a common area.

OSPF also allows and uses different area types. When deploying OSPF,

there must be a backbone area. Standard and stub areas connect to the back-
bone area. Following is a list of each area type, followed by a short descrip-
tion of that area type:

Backbone This area accepts all link-state advertisements (LSAs) and is
used to connect multiple areas.

Stub This area does not accept any external routing update, but it will
accept summary LSAs.

Totally Stub These areas are closed off from accepting external or sum-
mary advertisements.

Standard This is the normal area that accepts internal and external
LSAs, and summary information.

Not So Stubby This type of area is similar to Stub areas except that
Type 5 LSAs are not flooded into the area from the core. It can import
external AS routes into the area.

Move on now to learn the different types of link-state advertisements.

LSAs are the heart of OSPF's information exchange. Different types of LSAs
represent different types of route information. All of the defined and used
LSA types are summarized in Table 2.11.

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